1. FIELD OF THE INVENTION
The present invention relates to a rolling mill and rolling method, and more particularly to a rolling mill and a rolling method for metal strip, in which relatively small-diameter working rolls suitable for rolling hard, thin material are used.
2. DESCRIPTION OF THE PRIOR ART
A rolling mill for rolling metal strip, particularly hard, very thin material such a stainless steel, high carbon steel, spring steel and some alloy steels such as titanium alloy and high nickel alloy steels, uses small-diameter working rolls. Since such working rolls have too small a diameter for application of the rolling torque directly to them, there have been developed multiple-roll rolling mills such as the Sendzimir mill and other mills in which the drive is transmitted to the working rolls via one or more pairs of backup rolls through which drive is transmitted to the working rolls. Methods have also been developed for controlling the bending of the work rolls in such rolling mills, in order to achieve flatness of the product, by relative shifting of the backup rolls in the axial direction and also by applying vertical roll bending forces to the work rolls and the backup rolls.
The present invention is concerned with control of bending in the horizontal rolling direction, i.e. in the direction of travel of the material being rolled. This direction is referred to herein as the "horizontal direction" or "horizontal rolling direction" and these expressions do not include the axial direction of the rolls.
Since bending of the rolls in the horizontal direction increases with decrease of roll diameter, it imposes a limit on the reduction of roll diameter. The roll bending phenomenon in the horizontal direction is discussed more below.
U.S. Pat. No. 4,631,948 discloses a rolling mill in which drive is transmitted to the work rolls by backup rolls, and the work rolls are offset from the axial plane of the backup rolls in the horizontal direction. It is known that horizontal bending of the work rolls is reduced by offsetting the working roll axial plane from the backup roll axial plane in the direction downstream (in the rolling direction) of the backup roll plane because the frictional force applied by the backup rolls to the work rolls is then in opposition to the horizontal component of the rolling force (i.e. the force applied to the material being rolled by the work rolls). In U.S. Pat. No. 4,631,948, the work rolls are maintained in a fixed horizontal position in the mill frame, offset relative to the backup rolls, and are supported in the horizontal direction by rollers which contact the work rolls at portions thereof which are outside the region contacting the roll material but are of the same diameter as that region. The support rolls, which are on both sides of the work rolls in the horizontal direction, are forced against the work rolls hydraulically and serve to control horizontal bending, by applying bending forces to the rolls horizontally between their fixed bearing blocks. It is stated that the hydraulic cylinders which push the support rollers are independently controlled to produce the desired bending. However, in this mill because the bearing blocks are in a fixed horizontal position in the mill frame, appropriate control of the horizontal forces which vary in dependence not only on the rolling direction but also various other factors during rolling such as torque and rolling force, is not possible.
JP-A No. 63-60006 (1988) shows an arrangement closely similar to that of U.S. Pat. No. 4,361,948, in which again the bearing blocks of the work rolls are horizontally fixed.
JP-A No. 60-18206 (1985) shows a similar application of rollers to both sides of both ends of both work rolls, to provide horizontal support of the work rolls. In this case the rollers which are paired are applied by a mechanical adjustment system against the work rolls. All of the rollers are apparently adjustable in the horizontal direction, but there is no suggestion or control of the horizontal position of the work rolls which are shown with their axes in the vertical plane of the axes of backup rolls. It is stated that the journal bearings of the work rolls may be removed, presumably since all horizontal force is controlled by the rollers. No vertical roll bending is employed. If it is additionally required to apply vertical bending forces for vertical roll bending, it is not possible to remove the journal bearings since the vertical force must be applied through such bearings. In summary, this prior art disclosure suggests no solutions to the problems of control of vertical or horizontal roll bending.
JP-A No. 63-13024 (1988) in contrast describes the horizontal positioning of the work rolls by means of hydraulic cylinders acting on their bearing blocks. While this approach can restrict the amount of horizontal bending, by positioning the rolls appropriately for each rolling operation, it cannot supply any solution to the problem of out-of-plane bending in the horizontal direction arising from the relatively small resistance to bending of the small diameter work rolls.
Control of bending of the work rolls across the whole width of the work rolls is provided by a system of support rolls, such as in a Sendzimir rolling mill mentioned above. Another example is disclosed in U.S. Pat. No. 4,719,784, where a system of two support rolls in succession are carried by arms projecting from a support beam extending across the mill. The support beam carries a plurality of axially spaced rolling bearings providing horizontal support of the support rollers. The plane of the axes of the work roll, the two support rolls and the bearings is nearly horizontal. While such an arrangement provides good horizontal support of the work roll, it has the problem that the spaced bearings mark the support roll and these marks are transferred to the work roll, leading to transfer marking of the rolled product. Another problem is that the support rolls interfere with cooling of the work rolls.